Reciprocating piston and cylinder devices have been known and utilized for many decades as pumps and motors. These piston and cylinder devices are generally either single acting or double acting. In single acting piston and cylinder devices fluid under pressure is ported selectively to only one side of the piston in a forward stroke and the piston is returned by by non-fluid pressure means such as a return spring.
In double acting piston and cylinder devices fluid under pressure is ported to one side of the piston to drive it in a forward stroke and alternately to the opposite side of the piston to drive it in a return stroke. Usually a main directional valve is provided for porting fluid under pressure from a source alternately to two main passages connected to opposite ends of the cylinder.
The present invention is concerned with the manner in which the directional control valve is shifted to effect this alternate porting of fluid under pressure to the two main fluid passages connected to the cylinder. Frequently poppet valves are provided that control fluid pressure on the opposite ends of the main directional control valve. Mechanical trip rods driven by the piston are sometimes used to actuate these poppet valves alternately to provide an unbalance in fluid pressure acting on the opposite ends of the main valve to cause shifting. Such a poppet valve arrangement while satisfactory for many applications is subject to wear because the poppet valve actuation requires mechanical engagement, and it also creates noise resulting from impacting the trip rod mechanism with the poppet valves.
Such an arrangement is shown in Australian Patent No. 515,191. The air motor shown in this patent requires the added cost of two piston actuated poppet valves and a reset assembly to reset the main valve in case of malfunction. This design also needs a pair of tiny bleed ports in the main valve that necessitate a strainer in the air inlet to prevent foreign particles from clogging the ports.
Scienco Corporation manufactures an air motor model no. AP-30 that does eliminate these undesirable poppet valves by pneumatically shifting the main spool or directional control valve. In order to effect shifting of the main valve however it is necessary to provide two different size pistons (differential area) to create the differential pressure necessary for valve shifting and this required addition significantly increases the cost of the overall air motor.
It is the primary objection of the present invention to ameliorate the problems noted above in reciprocating piston and cylinder devices.